Automatic frequency control system utilizing a reference oscillator



Dec. 29, 1964 n.1. cARLsoN ETAL AUTOMATIC FREQUENCY CONTROL SYSTEM UTILIZING A REFERENCE OSCILLATOR 2 Sheets-Sheet 1 Filed Dec. 30. 1960 y NNN NN .QN NN ,M5 M NNN. NN NNN NN .www M .NNN NN NNN NN .mm/, /./.H NNN. NN .NNN Q www. A .NNN NN NNN N lp q NN NN NNN N ww f. NNN NN NNN N mm f l NNN NN NNN N l. NNN NN NNN .N QV N NNN NN NNN N. a NNN NN NNN m NNN .NN NNN N N NNN NNNNNNN NNNNNNNN NNN @ww @kf .DWQUNNMN @.WWQ SNQQN Qh'NN \\N RNNNNNNN N r NNNNNNN QN. ...NNN lv. NNNN. NN .NN N NNN NN a u ua ...NNANWWWKNN NMWMNNN A NN MQ 7 N/ H NNQ s wwvw v 100 s N N .M N. NN SN. r\\ QNNNNNNN uw. N MQNNNNNNNN Y NN. NNNNNE 1 NNN 1 .NN u 0 Il N NNN NN NN NNN DeC- 29, 1964 D. J. CARLSON ETAL 3,163,822

AUTOMATIC FREQUENCY CNTRO..Y SYSTEM UTILIZING A A REFERENCE OSCILLATOR Filed D66. 30. 1960 2 Sheets-Sheet 2 Q pedia/geil lsf/4%!! United States Patent Kce 3,163,322 4 AUTDMATC FREQUENCY 'CNTBQOL SYSTEM UTILIZWG A REFERENCE @SQIL-ATG? David Il. Carlson, Haddon Heights, NJ., and George B.

Waters, Philadelphia, Pai, assig'nors to Radio Corporation of America, a corporation of Deiawtue FiiedDec. 3i), 1969, Ser. No. 79,715 2 Claims. (Cl. 3259-421) This invention relates to the automatic control of tle tuning of a television receiver and particularly to an automatic frequency control system for the local oscillator of a super-heterodyne television receiver which system utilizes a single crystal-controlled device.

In addition to a VHF channel-selecting mechanism, many television receivers have a ne tuning control to provide the user with a more precise frequency control over the local heterodyne oscillator. In order to provide the optimum image on the television screen together With the optimum sound, it is necessary that the receiver local oscillator be adjusted so that the picture and sound carriers are located at the correct points in the intermediate frequency (1F) passband of the television receiver. This is especially true in `the tuning of a color television re'- ceiver. Not only must the picture and sound carriers be situated at their proper positions in the 1F passband, but also the color subcarrier must be properly positioned in the passband in order that the colors will be reproduced on the face of the kinescope Withthe properhue and saturation characteristics. It is desirable thatthe operator of a television receiver be freed of the requirement of making any line tuning adjustment of the local oscillator.

It is, therefore, an object of this invention to provide an improved automatic frequency control system for the local oscillator of a superheterodyne television receiver.

It is also an object of this invention to provide an improved automatic frequency control system for a television receiver tunable in the VHFtelevision band which system uses a single crystal-controlled device to produce the correct local oscillator frequency when theireceiver is tuned to any of the 12 VHF television channels.

These and other objects of the invention are eifected by using a single crystal-controlled device to control an automatic frequency control circnit for the local oscillator of the televisionlreceiver. s is known, theFederal Communications Commission (FCC) has established l2 VHF television channels for commercial television broadcasting and the frequency of the picture carriers Within each of these channelsY has been established. In accordance with the invention, the intermedi-ate frequency of the receiver, the local oscillator frequencies for all l-Z VHF channels and the frequency of a crystal-conf trolled reference oscillator are chosen so that tlie local oscillator fr'equemyV for each of the l2 VHF channels differs by' a fixed amount from a different liarnoliic of the crystal-contfoll'ed reference oscillator. Thus, a reference frequency from the crystal-controlled oscillator is mixed with the local oscillator frequency4 for any given channel and the difference frequency output signal of the mixer is applied to al detector circuit. li' the difference frequency deivates' from the fixed amount specified, the detector circuit generates a control signal which isA appliedV to a reactanc'e' device connected with' the tuningelements of the' local' oscillator to correct the' frequency' ofthe local oscillator' until the dineren'ce frequency is substantially equal to the fixed amount specified.

The invention Will be better understoodIl When: the' following: description is read in connection with theV accompanying drawings in which:

FIGURE 1iY isi-a block diagram of al television receiver havingfanf automatic frequency' control systeinvin accordance tWith'theinvention;

this invention, reference now is" Inade lo t 32,153,822 Patented Dec. 29, 1964 FIGURE 2 is a graph showing curves illustrating 'ciertain operational features of the circuit of FIGURE l;

FIGURE 3 is fable illustrating vSeveral other Opf'a.-` timiai features f the circuit of FIGURE 1 and;

FIGURE 4 is a scheinatic circuit diagramy (vvith cohveiiti'onal elements in block forni) of a televisionr'eceiver utilizing an automatic' frequency control circuit in accordance AWith one embodiment of the invention.

Referring ri'ow to 'dije drawings and iii particular to FIGURE 1, the ,signa-receiving and tuiiiiig apparatus foi a television receiver inclu es' an antenna to intei'ce'it and apply a television' signal to a radio frequency (RF) amplifier l2 which includes an inductivecapacitive tuning circuit 14. After the signal'is aniplied by the RF arnpliiier 12, it is applied to a mixer circuit 16 Where the radio frequency signal is miird vvith a local oscillator signal derived froin a local oscillator 1 8. lIt is to be understood that the RF amplifier 12, the mii/rer 16 and the local oscillator 18 may be conventional tuner yelements as exemplinedin RCA AServiceljata,1958-No. T10. Hetero'dyning between the local oscillator signal and the radio frequency signal rsiilt's in an intermediate frequency (IF) output signal from the iniirei' 16 which is applied to a 'television receiver 2 0. Details of the telvision receiver 20 have not been illustrated the interests of simplicity since they may be conventional and form noI part of the invention. It is' to be understood that such a television receiver includes intermediate frequency amplifier, a video detector, avideo amplifier, sound signal circuits and deilection control circuits a cathode ray picture reproducing tube; For an ,enibodimeiit of the invention iiiK a color television receiver', there will alsobe included color signal processing apparatus and circuitssucl as color demodulators, rriatriiiing amplifiers and the like. v

The local oscillator circuit 18 includes an inductivecap'active tuning element 2 2, much the' saine as the turiing circuit 14 for the RF amplifier 12. The local oscillator and RF ampliiier tuning vcircuits are ganged as in a conventional television receiver as illustrated by the broken line between the RF amplier' tuning element A14 and the local oscillator tuning element 22. A`ls`o, a television receiver is generally tunable discrete ysiepe Withinlthe VHF band' so' th'atthe ele'rnerits I4 and 22 .would norriany be tunable" in discrete steps frein channel to* channel.

In` accordance with the invention, a portion of the local oscillator signal, vvliich' eirists inL the oscillatorcir'cuit 18 and paniui'arly across the osciliato tuning element 22, ijs derived by suitable means such as a' loop 24 which is inductively coupled to the winductance component of the oscillator tuning element 24 and" is' applied to an FC mixer circuit 2 6; It is' to be understood that, alternatively, capacitive coupling may be used. A l's'o' applied' to the rminer circuit the otputof a 6 megacycletnic.) crystal-controlled reference oscillator circuit 28 which is richin harnionicfrequencies. The output of the `r'riixer iran isaarp'iieri faja 2me. manner circuit' su. A s' be morel fully' eiiplanied hereinafter, the locall oscillator circut'has suchk a frequency for each television channele that, at' correct tuning", lthe' dilference between eacl local ostinato@ frequency and a harmonic of' the 6 me. crystal oscillator is eXactly'Z rnc. 'Ile output` of the 2 rnc. ain-I pliei" is appliedv to a 2" mc; detector 32 and theoutput of the 2 mc.` detector isA applied to a reactancev circuit 34 Whichis connected: to control the frequency of the local oscillator" 1s.

In orde' to niol'efully understand the operation of the automatic frequency controli circuit in' accor 3f. 1 1; fais aut-,filigree noted die mima as signedE by' the FCC to each VHF television channel, the` local oscillator frequency for each channel, the harmonic frequency from the crystal-controlled reference oscillator which, when mixed with thecorrect local oscillator frequency, equals 2 mc. and the `detector output sign which is indicated as either positive or negative. The local oscillator frequencies have been chosen to produce an IF picture carrier of 44.75 mc. and a sound carrier of 40.25 mc. It will be noted that, for all television channels with the exception of channels and 6, a harmonic frequency of the reference oscillator is 2 mc. higher than the local oscillator frequency, but on channels 5 and 6 the reference oscillator harmonic frequency is 2 mclower than the local oscillator frequency. This is the meaning Vof the detector output sign and its signrilicancewill be more adequately explained hereinafter.

The response characteristic of the 2 mc. detector 32 Y than normal output signal from detector 32 in response is indicated in FIGURE 2. The output signal from the 2 d mc. detector of a particular frequency isa direct voltage. represented by the point on the characteristic curve corresponding `to that frequency. kThe solid curve 36 is the 2 mc. detector characteristic which is used when the local oscillator is tuned to all VHF channels with the exception of channels 5 and 6. The dotted curve 38 isthe 2 mc. detector characteristic which is used when the local oscillator is tuned to channels S and 6. It will be noted that these two characteristic curves of the 2 mc. detector are merely of reversed polarity. One means of accomplishing this polarity reversal will be disclosed when the circuit of.l FIGURE 4 is discussed hereinafter. `As will also be explained 1in the subsequent description of FIG- URE 4, the output signal from the 2 mc..detector may be superimposed on a xed bias for the reactance device. Such fixed bias is represented by the line 39 of FIGURE 2 and such response will be referred to in the following description as a normal output'signal from the detector, it being understood that such signal corresponds to va nosignal output from the detector itself.

In operation, the local oscillator is set to a particular channel. `Assume for example, that the localV oscillator is to be tuned to channel 3. From the table of FIGURE 3 it is indicated that the local oscillator frequency should be 106 mc. The local oscillator frequency is applied tothe i AFC mixer 26 where it beats with the 18th harmonic frequency of the 6 mc. reference oscillator 28 which is 108 mc. If thelocal oscillator frequency is exactly 106 rnc., the outputfrorn the AFC mixer 26 will be exactly 2 mc. VWhen this 2 mc. signal is applied to the 2 mc. detector 32, a normal output signal is derived therefrom and thus the reactance circuit 34 does not change'the local oscillator frequency.

However, if the local oscillator frequency is higher than 106 mc., the output'from the AFC mixer 26 will be less than 2 mc. and it is seen from the curve 36 of FIG- URE 2 that the output signal of the 2 mc. detector 32 Will be greaterthan normal. When applied to the reactance circuit 34,2this detector output signal decreases the local oscillator frequency until it -is the desir-ed 106 mc., atV which point the detector output signal is normal and nok further change of the local oscillator frequency is effected.

FIGURE 2 also indicates that, if the local oscillator frequency on channel 3 is too low and produces greater than a 2me. difference in the AFC mixer circuit 26, the output of the2 mc. detector 32 will be lessthan normal.

This decreased output fromy the 2 mc. detector, when -ap- Y Ylocaloscillator frequency produces an output signal'from Smixer 26 kWhichis greater than 2 mc.v As may IGURE 2, in order to obtain a greater to a frequency in excess of 2 mc., it is necessary to use the reversed'polarity output signal as represented by the broken line curve 38. The same is true if the frequency of the local oscillator l18 decreases'and a less than normal output signal from detector 32 is required, when channels 5 and 6 are selected.

It should be particularly noted that an automatic frequency control circuit in accordance with the invention does not derive its control signal through the intermediate frequency amplifier of the television receiver, but rather controls the local oscillator frequencydirectly from the local oscillator signal itself. Thus, various signals that may be present in either the RF or IF amplifiers of the receiver, cannot affect either the stability ofthe system or cause blocking of the system by a spurious signal. automatic frequency control system is effectively isolated ,from all signals within the signal transmission channel of relatively low frequency where there is a minimum driftV in the value of the components used in the amplier circuit. v There is also no switching from one reference frea quency to another required of the automatic `frequency control system, since this is automatically determined by the particular reference oscillator harmonic which will beat with the local oscillator signal. l

Referring now to FIGURE 4, a particular embodiment of the'invention is illustrated and includes an antenna 10,

an RF amplier 12, a mixer circuit 16, a local oscillator circuit 18,. and a television receiver 20 as described in connection with FIGURE 1. `The block elements of FlG UREl also are indicated and designated by the same by connecting the anode 42 through an anode resistor 44 to a kfirst source of positive polarity B1 and the cathode 46 is connected through a cathode resistor 48 to a second source of positive polarity B2 which is of less magnitude 72; Theinductor 60 is so illustrated to indicate that the thanpthat of source B1. Suitable bias is supplied to the control grid 50 of the tube through a resistor 52 con-l nected between the cathode 46 and the control grid 50, thecathode and control grid being connected to ground through capacitors 54 and 56 respectively. The ,fre-z quency determining elements of the oscillator circuit are connected between the Yanode 42 and the control grid 5 0 and include a variable capacitor 58 connected directly between the anode 42 and the control grid 50 and a tapped inductor 60 which has one extremity connected to an anode terminal 62 and thence through an inductor 64 and a capacitor 66 to the anode 42 and the other extremity connected-to a grid terminal 68 which is directly connected tothe control grid 50.y The tap of the inductor 60 is connected to anjoutput terminal 70 vwhich is in turn connected to the mixer 16 through a coupling capacitor local oscillator 18 is tunable; to any of the local oscillator frequencies shown in the table of FIGURE 3 so that the television receiver may be tuned Vto anyone of the 12 f Y VHF television channels.

In accordance with the invention, a pickup loop 74.is

placed within .the oscillator compartment 18 and is con-1' Vnected to 4apply a portion of the oscillator signaltov thea Y The controlgrid-.76 of a 6' me.` reference oscillator tube-78. The reference oscillator tube- 78v isi connected as a conventional` Pierceoscillator. with a, dmc-f. crystal 81k connected in series-with acapacitor 82-.andaninductor 84 between/the anode andcontrol grid of the tube-78. A small capacitor 86 may be connected acrossthecrystal 80 to assure proper capacitive loading-on. the crystal. The inductor 84 istv inserted to insure that-the reference oscillator output-signal is rich intha-rmonics.

The-.anode ofthe reference,` oscillatortube 7&is. connected through a coupling capacitor 88 to the control grid of a mixer-tubeO-to apply both the reference oscillator signal and the. local -oscillator-signal-thereto.y The anode of the mixer tube Wis-,connected toen-inductivecapacitive circuit 92 .which-is tuned to 2-mc. and -the output signal of the mixer tube Yisdeveloped thereacross and is: applied bye-coupling capacitor 94 -to' the 2 rnc. amplifier 30 which may be of-'a-conventional-desi-gn.

The output of the 2 me. yallflliliery 30 is` applied to the tuned primary 96i-of. a ,transform'er 98,` thesecondary Winding 1000i which has its extremity connected to the anodes. of a'pair of crystal retainers-102 and 104.` The usual reference signal. is applied-tothe secondary of the transformer by a capacitor 10d-connectedl between one terminal of the-primary 96 and the center tap of the secondary/100. A. pairof: resistors-108-and T10-are con- -nected between the cathodes? of the crystal rectiliers 102 and 11M-andthe cathodes-are .connectedVA respectively to ground-- by a= pairof radio frequency bypass capacitors 112 and 111-4. Inorder'to-complete the 2- mc.. detector circuit, the junction of the resistors 108zand11f10isconnected directlyto the center'- tap of the secondary winding 100 of the transformer-'98;

The cathodes of the crystal-reotiers-102and. 1.04 are connected to a polarity reversing switch 116. In one positionofthe switch thecathode of` rectifier 102 is connected through a resistor 118 to the cathode of a reversed bias reactanceV diode 120n which is utilizedas a vari-able capacitive. reactance control de vice for, the local oscillator1 tube 40, to the grid terrrnalS-ofiwhich it is-.conne'cted through a1 capacitor 12,2. Also, in thatone switch position the cathode of the rectifier 1042 is` connected tov a battery 124v which serves as a source of fixed` reverse bias for the reactance diode 120. Infthe othen position of the switch 116 the connections. of. the rectifiers 102 and-104 to the reactance diode 120 andthe battery 124 are reversed.

More specifically, the polarity reversing switch cornprises a pair of common terminals 126 and 128 lto which the respective cathodes of the diodes 102 and 104 are connected. The switch also consists of an upper pair of contacts 130 and 130 respectively for the common terminals 126 and 12S and to which the common terminals are normally connected. In the normal switch position -as shown, the biasing battery 124 is connected through the upper contact 130 to the common terminal 126, thence through the resistors 110 and 108 of the detector and through the common terminal 128 and upper contact 130 of switch 116 to the reactance diode 120. The common terminals 126 and 128 also have respective lower contacts 132 and 132 to which they are connected for -channels 5 and 6 oper-ation of the receiver.

It will be noted that, in this switch position, the. biasing battery 124V is connected through the lower Contact 132 to the common terminal 128, thence through the two resistors 108 and 110 of the detector and through the common terminal 126 and lower contact 132 of switch 116 to the reactance diode 120. In this latter switch Y position, the polarity of the detector output is reversed from that obtained by the normal switch position to provide the proper correction voltage to the reactance diode as indicated by the curve 38 in FIGURE 2. The movableV 6. such as-by a simple cam arrangement linked to thechannel selecting apparatus.

Thus, inoperation, the signal-from the local oscillator 18- picked up by the pickup loop 74, together with the reference oscillator harmonic signalsfrom the-reference oscillator, are applied to the control grid of the` mixer tube 90. The tuned-circuit 92- selectsthe output of the mixer thatis nearor at-2 mc. and applies this signalto the detector through-the 2me. amplifier 30. If the frequency ofthe signalis exactly-2me., as when the receiver is correctly tuned, a normal-output' signal is derivedfrom the detector and. the-reverse-bias voltage across the reactance diode is` determined fully by the battery 124 connected through the polarity reversing switch116 as described.. If thereceiver is incorrectly tuned, the fgrequency-of the signal applied to the detector deviates from 2 mc. In this case an output signal component is developed across the detector resistors 108'.l and 110. This output signal has a polarity dependent upon the sense of the deviation from 2- me. and an amplitude dependent upon the magnitude of the devi-ation. The-combination of this outputsignal with-the xedbi-asing voltage of the battery 124 either increasesor decreases the reverse-biasing of the reactance diode 120 which, in-turn, changes the frequency of the local-oscillator suitably to correctly. tune the receiver.

While the detector 32; has been illustrated as a conventional discriminatorcircuit, it should-be noted that a portion of the'reference oscillator signal.- at 6i mc. may be derived from the oscillator tube '7S and applied directly at a three-to-onecount ,clowny circuit toderive a 2 mc. signal which may be used as one signal -for aproduct or phase` detector in place of the discriminatorshown.

Having thus described the invention, what is claimed is:

1. In an automatic frequency4 control system for a superheterodyne television receiver, in combination,

channelf selecting4 means for; selecting any one of a plurality of television signal transmission channels, each. of-y saidchan-nels corresponding respectively to a different radio frequency signal, the radio frequenciesof a first` group of said channels differing from one anothery by av fixed frequency or multiple thereof; andthe radiofrequencies of a second-group of said;r channels differ-ring from-,the radio frequencies of saidv first group ofi channels by. frequencies, other than said fixed frequency or multiples thereof; a variable frequency local oscillator;

means coupled to said channel selecting means and said oscillator for adjusting the frequency of said local oscillater at least approximately to that required for proper `receiver response for said selected channel; e a crystal-controlled reference oscillator producing a signal of said fixed frequency and a plurality of signals at harmonic frequencies of said xed frequency, said fixed frequency being selected so that the frequency ofone of said plurality of signals diifers by-a given nominal frequency from the local oscilla- -tor frequency required for proper receiver response to each of said plurality of television signal transmission channels, sad signals at harmonic vfrequencies having higher frequencies than said. local oscillator frequency for one of said groups of channels and having lower frequencies than said local oscillator frequency for the other of said groups of channels; a frequency mixer; means coupling said local oscillator and said reference oscillator to said mixer to produce an output signal having said nominal frequency and which only devi-ates fromsaid nominal frequency to the extent that said local oscillator frequency may dilfer from that required for said proper receiver response; means coupled to said mixer'for detecting any deviap tion in said output signal from said nominal frequency so -as to produce opposite polarities of a control signal, each representative of the magnitude n of said deviation;

reactance circuit means including a variable capacitance diode and responsive to said control signal for adjusting the frequency of said local oscillator precisely to that required'for `the transmission channel selected by said channelselecting means; Y Y

and means including a reversing switch coupled to said channel selecting means and connecting said detecting means to said reactance circuit means to impress upon said diode a selected polarity of said control signal dependent Yupon which one of said local oscillator and harmonic frequencies is the higher in the production of said output signal.

2. In an automatic frequency control system for a superheterodyne television receiver, in combination,

channel selecting means for selecting any one of a plurality of television signal transmission channels,

each of said channels corresponding respectively to" said oscillator for adjusting the frequency of said local oscillator` at least approximately to that required for proper receiver response for said selected channel;

a crystal-controlled reference oscillator producing VaV signal of lsaid fixed frequency Vand a plurality of signals at harmonic frequenciesvof said fixed frequency,

said fixed frequency being selected so that the fre-A quency ofv one-of said plurality of signals differs Y by a given nominal frequency from' the local oscillator frequency required for proper receiver response to each of said plurality of television signal triansmissionV channels, said signals at harmonic frequencies having higher frequencies than said lo-V cal oscillator frequency for one of said groups of channels and having lower frequencies than said local oscillator frequency for the other of said groups of channels;

a frequency mixer;

means coupling said local oscillator vand said reference oscillator to said mixer to produce an output signal having said nominal frequency and which only deviates from said nominal frequency to the extent that said local oscillator frequency may differ from that required for said vproper receiver response;

means coupled to said mixer for detecting any deviation in said output ,signal from said nominal frequency so as to produce opposite-'polarities of a control signal, each representative of the magnitude of said deviation; Y ,K

reactance circuit rmeans including a variable capacitance diode and responsive to said control signal for `ad-v i in like polarities in one position of said switch for television channelsV where said local oscillator frequency is higher by said nominal frequency than va harmonic frequency of said fixed frequency in the.

production of said output signal, and in opposite polarities in the other position of said switch for selected television channels Where said local oscillator frequency is lower by said nominal frequency than a harmonic frequency of said fixed frequency in thel production of said output signal.

References Cited in the file of this patent ,Y

UNITED STATES PATENTS 2,555,391 Bach June 5, 1951 2,595,608 Robinson et al. May 6, 1952 12,685,032 Cox July 27, 1954 2,704,329 Law f.-- Mar. 15, 1955 2,810,832 Broadhead Oct. 22, 1957 2,855,506 Schabauer Oct. 7, 1958 v2,860,246 Jakubowics Nov. 1l, 1958 2,936,428 196() VSchweitzer May 10, 

1. IN AN AUTOMATIC FREQUENCY CONTROL SYSTEM FOR A SUPERHETERODYNE TELEVISION RECEIVER, IN COMBINATION, CHANNEL SELECTING MEANS FOR SELECTING ANY ONE OF A PLURALITY OF TELEVISION SIGNAL TRANSMISSION CHANNELS, EACH OF SAID CHANNELS CORRESPONDING RESPECTIVELY TO A DIFFERENT RADIO FREQUENCY SIGNAL, THE RADIO FREQUENCIES OF A FIRST GROUP OF SAID CHANNELS DIFFERING FROM ONE ANOTHER BY A FIXED FREQUENCY OR MULTIPLE THEREOF AND THE RADIO FREQUENCIES OF A SECOND GROUP OF SAID CHANNELS DIFFERING FROM THE RADIO FREQUENCIES OF SAID FIRST GROUP OF CHANNELS BY FREQUENCIES OTHER THAN SAID FIXED FREQUENCY OR MULTIPLES THEREOF; A VARIABLE FREQUENCY LOCAL OSCILLATOR; MEANS COUPLED TO SAID CHANNEL SELECTING MEANS AND SAID OSCILLATOR FOR ADJUSTING THE FREQUENCY OF SAID LOCAL OSCILLATER AT LEAST APPROXIMATELY TO THAT REQUIRED FOR PROPER RECEIVER RESPONSE FOR SAID SELECTED CHANNEL; A CRYSTAL-CONTROLLED REFERENCE OSCILLATOR PRODUCING A SIGNAL OF SAID FIXED FREQUENCY AND A PLURALITY OF SIGNALS AT HARMONIC FREQUENCIES OF SAID FIXED FREQUENCY, SAID FIXED FREQUENCY BEING SELECTED SO THAT THE FREQUENCY OF ONE OF SAID PLURALITY OF SIGNALS DIFFERS BY A GIVEN NOMINAL FREQUENCY FROM THE LOCAL OSCILLATOR FREQUENCY REQUIRED FOR PROPER RECEIVER RESPONSE TO EACH OF SAID PLURALITY OF TELEVISION SIGNAL TRANSMISSION CHANNELS, SAID SIGNALS AT HARMONIC FREQUENCIES HAVING HIGHER FREQUENCIES THAN SAID LOCAL OSCILLATOR FREQUENCY FOR ONE OF SAID GROUPS OF CHANNELS AND HAVING LOWER FREQUENCIES THAN SAID LOCAL OSCILLATOR FREQUENCY FOR THE OTHER OF SAID GROUPS OF CHANNELS; A FREQUENCY MIXER; MEANS COUPLING SAID LOCAL OSCILLATOR AND SAID REFERENCE OSCILLATOR TO SAID MIXER PRODUCE AN OUTPUT SIGNAL HAVING SAID NOMINAL FREQUENCY AND WHICH ONLY DEVIATES FROM SAID NOMINAL FREQUENCY TO THE EXTENT THAT SAID LOCAL OSCILLATOR FREQUENCY MAY DIFFER FROM THAT REQUIRED FOR SAID PROPER RECEIVER RESPONSE; MEANS COUPLED TO SAID MIXER FOR DETECTING ANY DEVIATION IN SAID OUTPUT SIGNAL FROM SAID NOMINAL FREQUENCY SO AS TO PRODUCE OPPOSITE POLARITIES OF A CONTROL SIGNAL, EACH REPRESENTATIVE OF THE MAGNITUDE OF SAID DEVIATION; REACTANCE CIRCUIT MEANS INCLUDING A VARIABLE CAPACITANCE DIODE AND RESPONSIVE TO SAID CONTROL SIGNAL FOR ADJUSTING THE FREQUENCY OF SAID LOCAL OSCILLATOR PRECISELY TO THAT REQUIRED FOR THE TRANSMISSION CHANNEL SELECTED BY SAID CHANNEL SELECTING MEANS; AND MEANS INCLUDING A REVERSING SWITCH COUPLED TO SAID CHANNEL SELECTING MEANS AND CONNECTING SAID DETECTING MEANS TO SAID REACTANCE CIRCUIT MEANS TO IMPRESS UPON SAID DIODE A SELECTED POLARITY OF SAID CONTROL SIGNAL DEPENDENT UPON WHICH ONE OF SAID LOCAL OSCILLATOR AND HARMONIC FREQUENCIES IS THE HIGHER IN THE PRODUCTION OF SAID OUTPUT SIGNAL. 